Undervolt your CPU, both the CPU and GPU share a heat pipe so a change in one's temperature will affect the other.
Also you can use Asus' GPU Tweak II to set a temperature limit for your GPU, but It'll also curb it from boosting to high frequencies.
+ Always play your games with the fans set to max speed so it can help cool down the CPU and GPU.
Finally if none of the options work out for you, you can always repaste your GPU and CPU with something good like the Thermal Grizzly Kryonaut.
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